1. Field of the Invention
This invention relates to a method of killing microorganisms in the inside of a container. More particularly, this invention relates to a method in which the inside of such container is exposed to a plasma induced by focusing a high-power laser beam in an electromagnetic field.
2. Prior Art
Killing of microorganisms, which when carried to totality constitutes sterilization, in containers into which are filled such substances as parenteral and other medications, foods, beverages, dairy products, and the like, has been practised for decades for the purpose of preventing the transmission of disease. Many methods have been devised to accomplish this purpose. Heat, both dry and wet, has been a popular method of killing microorganisms in the food, beverage and pharmaceutical arts for a long time. The use of chemicals such as formaldehyde, phenol, ethanol, ethylene oxide, and the like for killing microorganisms has found many useful applications. More recently irradiation, such as beta, gamma, and ultraviolet rays have been employed in specialized applications for killing microorganisms.
In 1968, U.S. Pat. No. 3,383,163 described a method of sterilizing the surface of a material which does not conduct electricity comprising contacting such surface with a gaseous plasma at an extremely high temperature. In this method a corona discharge was utilized to generate a plasma inside a container. The corona discharge was achieved by introducing a grounded electrode into the container, surrounding the container with a coil and pulsing from about 5000 to about 7000 volts and above into the coil. Exposure of the surface to a plasma for a very brief period of time, normally not longer than one-tenth of a second, is described.
In the interval since U.S. Pat. No. 3,383,163 was issued, many attempts have been made to develop the plasma sterilization process into an economically feasible method because of the inherent advantage of killing and the microorganisms in the inside of a container just prior to filling. However, the mechanical problems associated with introducing a grounded electrode into a container and simultaneously surrounding the container with a high voltage coil have been found to be of such a magnitude as to defeat exploitation of the invention. Moreover, the volume of plasma generated by the corona discharge is dependent on the style and shielding of the electrode tip, the winding of the high voltage coil and the potential difference between said electrode and said coil at the moment of the pulsed discharge, and such requirements have presented problems in the location of the electrode and coil so as to fill the container with plasma. Furthermore, the voltage required to initiate the corona discharge is substantial and requires specialized electrical circuitry.
U.S. Pat. No. 3,955,921 describes a novel method of killing microorganisms inside of a container by repeatedly sparking an ultra-short-pulsed laser beam in the inside of the container. Each spark resulted in an ultra-short-lived plasma. This method has the advantage of inducing the microorganism killing plasmas within the container without the need for the mechanical introduction of an element into the containers. Moreover, microorganisms were killed in the inside of the container without contacting the inside surfaces of the container with the plasma. However, the need for a succession of independently generated plasmas extended the time which each container had to be in position for the laser beam to be focused thereinto.
Accordingly, it is an object of this invention to provide a method of killing microorganisms in the inside of a container with a continuous plasma that is induced by a single pulse of a focused high-power laser beam and is expanded and sustained inside such container for whatever interval is required to effect a complete killing of all microorganisms therein.